To date, with an increase in power demand placed on power distribution systems and the introduction of distributed power sources, replacement of transformers with large-capacity transformers has been inevitably required. Accordingly, an increased fault current exceeds the capacity of a circuit breaker installed in the system, so that research into a superconducting fault current limiter (SFCL) has been conducted as a realistic scheme for improving the stability of the system and reducing economic cost stemming from replacing the circuit breaker with a large-capacity device.
FIGS. 1 to 3 are diagrams showing examples of a conventional SFCL. The SFCL shown in FIGS. 1 to 3 uses magnetic coupling between two coils (a primary winding and a secondary winding) wound around the same core. The form of coupling between the primary winding and the secondary winding corresponds to parallel connection of a non-isolated type shown in FIG. 1, series connection of a non-isolated type shown in FIG. 2, and an isolated type shown in FIG. 3. Before a fault occurs, a High Temperature Superconductor (HTSC) maintains zero resistance in a superconducting state, and magnetic flux components generated by the two coils cancel each other, so that voltages induced in the respective coils become 0. However, when a fault occurs, and the current flowing through the HTSC exceeds a threshold, resistance is generated in the HTSC, and then magnetic flux components generated in the two coils do not cancel each other any more. Accordingly, voltages are induced in the respective coils, and impedance is generated in the SFCL, so that the fault current is limited.
Such an SFCL using magnetic coupling between two coils is characterized in that a burden on power caused by a fault can be divided into and exerted on the two coils, which are connected in a non-isolated type (in parallel or in series) or in an isolated type, and the HTSC, thus not only reducing the number of HTSCs, but also inducing a phenomenon (quench) in which series-connected HTSCs simultaneously make a phase transition to phase conductors because of magnetic coupling. Further, a current limiting magnitude can be effectively adjusted by controlling the intensity of the impedance through the adjustment of a turns ratio. However, since the problem of a drop in a bus voltage arises when a fault occurs, a method capable of solving such a voltage drop problem is required.